The invention relates to an optical switching device comprising a switching film which contains a metal compound. The invention also relates to an electrochemical switching device comprising such a switching film. The invention additionally relates to applications of such a switching device.
In the relevant switching devices, the optical properties are governed by external influences, such as gas pressure, an electric potential or current.
For example, electrochromic devices are well-known, in which a layer of an electrochromic material, such as MoO.sub.3, is sandwiched between two transparent electroconductive electrode layers, for example, of indium-tin oxide. A layer of an H.sup.+ - or Li.sup.+ -ion-conducting material is present between an electrode and the electrochromic material. The device also comprises an ion-storage layer for storing said ions. The application of an electric potential of several volts across the electrodes causes the transmission or colour of the layer stack to change. Said transmission change is reversible. Electrochromic materials are used, for example, in variable-transmission windows for buildings and anti-dazzle mirrors in cars.
A drawback of oxidic electrochromic devices is that an extensive layer stack is required for their operation. A further important disadvantage is that such materials enable only a relative small transmission change, and hence a small contrast, to be attained.
In the non-prepublished international patent application wo 96/38759 filed by Applicants, a switching device is described in which a film of gadolinium hydride GdH.sub.x can be reversibly switched between a low-hydrogen (x&lt;.about.2) composition and a supersaturated high-hydrogen (x&gt;.about.2) composition by an exchange of hydrogen. Both compositions have different optical properties. At a low hydrogen content (x&lt;.about.2), the film has a metallic character, and is non-transparent. At a high hydrogen content (x&gt;.about.2), the film is semiconductive, transparent and yellow in the transmission mode. Switching can be carried out at room temperature.